The Homerun Per Contact Rate (Part 2)

Last week I found the all-time leaders in HRs per time making contact. This week I look at the overall rate in MLB since 1955 for non-pitchers only. Times making contact is AB - K + SF. The first graph below shows this rate. Then then next one shows HRs per AB (HR%) and then the next one shows HRs per plate appearance with PA = AB + non-intentional walks + SF + HBP. The last graph shows strikeouts per AB. I don't show strikeouts per PA since that mimics strikeouts per AB very well.

HRs per AB hit a low of 1.75 per 100 ABs in 1976 and reached a high of 3 in 2000. So the absolute gain from 1976 to 2000 was 1.75 and the ratio of the later rate to the earlier rate was 2.

HRs per time making contact a low of 2 per 100 times making contact in 1976 and reached a high of 4.22 in 2000. So the absolute gain from 1976 to 2000 was 2.22 and the ratio of the later rate to the earlier rate was 2.11.

HRs per PA hit a low of 1.57 per 100 PAs in 1976 and reached a high of 3.08 in 2000. So the absolute gain from 1976 to 2000 was 1.51 and the ratio of the later rate to the earlier rate was 1.96.

Why would the HR per contact rate rise more than the either of the other two rates in both absolute and relative terms? One thing comes to mind is that players are swinging more for HRs, which would raise their K total but that hard swinging would lead to more HRs when they do make contact. Maybe it was steroids or smaller parks or a livelier ball, but those would raise the HR per AB rate, too but why would those things raise the HR per contact rate even more?